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TA Instruments AR-G2 Rheometer

To investigate clathrate hydrate slurries a commercial TA Instruments AR-G2 rheometer equipped with a pressure cell is utilized. The pressure cell is a constant volume cell with a concentric cylinder geometry (cup radius 14 mm and bob radius of 13 mm) capable of operating at pressures of up to 13.8 MPa. The pressure cell is also equipped with a Peltier cooler with a temperature range from -10 to 150° C. The AR-G2 is a controlled stress rheometer capable of conducting dynamic or steady shear tests over time. The dynamic oscillation tests can be preformed at a frequency range from 7.5x10−7 to 628 rad/s. The shear tests can be run with an angular rotation 1.4x10−19 to 300 rad/s with torque sensitivity from 0.003 μN/m up to 200 mN/m. However, when the pressure cell is attached the sensitivity drops to approximately 100 μN/m.

Shear and oscillatory time sweeps experiments are possible with this rheometer. Shear time sweeps are conducted by running the rheometer at a constant shear rate (or speed). During the entire test torque measurements are recorded (every 10 seconds) and are report as effective viscosities. These measurements are used to detect clathrate hydrate nucleation and changes in the sample over time as it was sheared. Shear ramps are conducted by starting at a low shear rate and increasing the shear rate. Again torque measurements are recorded and reported as effective viscosities. These shear ramps are done on a much shorter time scale than the time sweeps, helping to eliminate convoluting effects. From these measurements information such as shear thinning can measured and yield stresses can be estimated. For oscillatory time sweeps, the rheometer is oscillated at a constant frequency and constant strain. The torque responses are measured over time, reported as the elastic (G’) and loss moduli (G”). These tests are used to detect clathrate hydrate nucleation and changes in the sample over time, while under relatively quiescent conditions.